The present invention generally relates to the processing of metal-contaminated, precious metals-containing solutions, and more particularly to a method in which gold ore that also contains copper and other contaminant metals is treated to effectively separate gold from the copper and other contaminant metals.
To recover elemental gold (Au) from gold-containing ore, the ore is typically contacted with one or more aqueous cyanide-containing leach solutions (or lixiviant). The gold and other metals are dissolved in the solution, forming various metal cyanide complexes such as Au(CN)2xe2x88x921 and Cu(CN)3xe2x88x922.
A variety of different physical methods may be employed to contact the ore with the cyanide-containing leach solution. Two common methods are heap leaching and vat leaching. In heap leaching, coarsely comminuted ore is placed in a pile which is positioned on an impervious liner. The cyanide-containing leach solution is applied to the top of the ore pile and allowed to travel (e.g. percolate) through the heap. A pregnant leach solution containing one or more monovalent gold-cyanide complexes and other dissolved metal cyanide complexes collects on the liner at the bottom of the pile. In vat leaching, finely comminuted ore is placed in a large container or xe2x80x9cvatxe2x80x9d along with the cyanide leach solution to form a slurry. The solution extracts gold and other metals from the ore forming the pregnant leach solution.
A number of different procedures may be employed to recover the dissolved gold from the cyanide solution. Two common gold recovery methods are the Merrill-Crowe process and the activated carbon process.
In the Merrill-Crowe Process, the pregnant leach solution undergoes zinc cementation/precipitation reaction. Specifically, the pregnant leach solution containing the gold-cyanide complex is combined with elemental zinc (Zn) to generate solid elemental gold (Au) which resides within a gold-zinc solid sludge reaction product. The product is removed by filtration from the residual liquid fraction (which consists primarily of free cyanide ions [(CN)xe2x88x92] and a dissolved Zn(CN)4xe2x88x922(aq) complex). The product is processed to isolate and recover the elemental gold by combining the product, after water washing, with sulfuric acid (H2SO4) in the presence of air to dissolve excess (unreacted) elemental zinc and other metals including copper and cadmium. The remaining solid material is smelted in the presence of a flux to produce a highly pure gold dore.
In the activated carbon process, the pregnant leach solution is placed in contact with activated carbon and the dissolved gold-cyanide complexes in solution are adsorbed onto the surface of the activated carbon. After adsorption, the gold-containing carbon product is filtered to remove residual xe2x80x9cbarrenxe2x80x9d liquid, followed by xe2x80x9cdesorptionxe2x80x9d or removal of the gold-cyanide complex from the xe2x80x9cloadedxe2x80x9d activated carbon (e.g. the gold-containing carbon product) by passing an eluant solution through the carbon. It is theorized that cyanide ions [(CN)xe2x88x92] in the eluant solution effectively replace/exchange the adsorbed aurocyanide ions (gold-cyanide complex) which are released into the eluant solution. The resulting gold-containing eluant product (which contains the desired gold species [aurocyanide ions/gold cyanide-complex]) is then processed by any suitable technique to recover elemental gold.
Regardless of which methods are ultimately used to obtain elemental gold from gold-cyanide complexes, numerous technical and economic problems can occur when gold ore is processed which contains substantial amounts of elemental copper and other contaminant metals. Such metals can have a stronger affinity for cyanide ions than gold and form metal cyanide complexes. For example, the copper-cyanide complex (Cu(CN)3xe2x88x922) which is generated as a result of this reaction is incapable of extracting gold from gold ore to yield the desired gold-cyanide complex and consumes three moles of (CN)xe2x88x92. As more copper leaches into the recirculating leaching solution (which occurs during reuse of this material and repeated passage thereof through incoming quantities of gold ore), increasingly large amounts of cyanide are lost to this complex. Such contaminant metals can therefore cause excessive cyanide consumption, thereby increasing process operating and capital expenses and substantial reductions in the operating efficiency of the entire gold production facility.
In addition to excessive cyanide consumption, copper and other metals within the gold ore can also result in an increasingly impure elemental gold product. Additional and more costly refining procedures must therefore be employed to solve this problem. By way of example, if the Merrill-Crowe process is used, extraneous copper materials in the solution can dramatically reduce the precipitation efficiency of the system by causing zinc passivation, with the term xe2x80x9cpassivationxe2x80x9d involving a process in which the zinc is rendered non-reactive to the gold-cyanide complex which prevents the gold precipitation process from taking place. Additional zinc is often required which again increases overall production costs. Excessive contaminant metal contamination of the leaching solution can also reduce the operating efficiency of the smelting process associated with this embodiment by causing prolonged smelting times. For example in systems which employ the activated carbon process, copper materials (e.g. copper-cyanide complexes) will substantially inhibit the functional capabilities of the activated carbon, thereby xe2x80x9cfoulingxe2x80x9d this material and causing increased carbon consumption. Power consumption is likewise increased in subsequent electrowinning stages if many contaminant metals are not removed from the system.
It is an object of the present invention to provide a method for separating gold and/or silver from copper and other contaminant metals in a gold and/or silver processing system which enables the removal of copper and/or other contaminant metals from the system.
It is another object of the invention to provide a method for separating gold and/or silver from copper and other contaminant metals in a gold and/or silver processing system which enables the purity levels of elemental gold and/or silver to be relatively high.
It is a further object of the invention to provide a method for separating gold and/or silver from copper and other contaminant metals in a gold and/or silver processing system in which the removal of copper and other contaminant metals is accomplished with relatively low consumption of cyanide and other reagents so that the overall efficiency of the system is improved.
The claimed process overcomes the problems outlined above in a very effective manner which will become readily apparent from the detailed information presented below. While specific processing systems and gold and/or silver recovery technologies will be discussed in connection with the claimed procedure, the present invention shall not be limited to any particular cyanide-based gold and/or silver extraction method or to leaching solutions generally. Instead, the invention is prospectively applicable to any production system which places gold- and/or silver-containing materials in physical contact with solutions containing free cyanide ions [(CN)xe2x88x92] so that a gold- and/or silver cyanide complex as defined above is generated as well as any other application in which gold- and/or silver-containing solutions contaminated by other metals are treated to recover the gold- and/or silver. For example, the processes of the present invention are applicable to electroplating solutions.
In one embodiment of the present invention, a process for recovering a dissolved monovalent precious metal cyanide complex (e.g.,Au(CN)2xe2x88x921) from a cyanide solution containing the dissolved monovalent precious metal cyanide complex and one or more dissolved multivalent metal cyanide complexes (e.g., Cu(CN)3xe2x88x922)is provided. A monovalent precious metal cyanide complex is a complex formed by gold or silver with two or more cyanide ions and therefore has an overall charge having an absolute value of one. A multivalent metal cyanide complex is a complex formed by a metal other than gold or silver with three or more cyanide ions and has an overall charge having an absolute value of two or more. The process includes the steps of (a) passing the cyanide solution through a filter to form a retentate containing a portion of the dissolved monovalent precious metal cyanide complex and most of the one or more dissolved multivalent metal cyanide complexes and a permeate containing most of the dissolved monovalent precious metal cyanide complex and (b) thereafter recovering the precious metal from the permeate to form a precious metal product. The retentate does not pass through the nanofiltration membrane while the permeate passes through the membrane. The permeate typically contains more than about 50% of the dissolved monovalent precious metal cyanide complex but less than about 50% of the dissolved multivalent metal cyanide complex. In contrast, the retentate typically contains more than about 50% of the dissolved multivalent metal cyanide complex but less than about 50% of the monovalent precious metal cyanide complex. The claimed method thus effectively removes undesired multivalent metal cyanide complexes (e.g. metal cyanide complexes in which the metal is copper, zinc, cobalt, iron, calcium, magnesium, nickel, lead, cadmium, mercury, platinum, and palladium) at the early stages of production in a rapid and efficient manner.
The cyanide solution can be formed by any number of processes. Commonly, the solution is formed by contacting a precious metal-containing material with an aqueous cyanide solution (e.g., a lixiviant) to extract the metals content of the material into the solution. The cyanide-containing solution shall be defined to encompass a solution, preferably aqueous, comprising free cyanide ions [(CN)xe2x88x92] therein in combination with a selected counter-ion (e.g. Na+, K+, Ca+2, and the like). Representative cyanide solutions suitable for this purpose will generally contain a dissolved cyanide compound (salt) therein, with representative examples of this material including but not limited to sodium cyanide (NaCN), potassium cyanide (KCN), calcium cyanide (Ca(CN)2), ammonium cyanide (NH4CN), organic alpha-hydroxy cyanides (e.g. lactonitrile), and mixtures thereof. The liquid product will typically include about 1xc3x9710xe2x88x923-1xc3x9710xe2x88x924% by weight gold-cyanide complex and about 0.05-1.0% by weight multivalent metal cyanide complexes though these values are subject to change in accordance with the particular type, grade, and character of material being processed.
The filter can be any suitable filtration device that is capable of removing selectively the desired multivalent metal cyanide complexes from the solution. Preferably, the filter has a pore size ranging from about 5 to about 100 angstroms and more preferably from about 10 to 20 angstroms. Preferred filters include electrically charged filters which generally repel dissolved multivalent metal cyanide complexes while passing dissolved monovalent precious metal cyanide complexes, with a xe2x80x9cnanofiltration-type membranexe2x80x9d being more preferred.
As will be appreciated, the conditions under which the separation is effectuated are important to the efficiency of the separation. In a preferred embodiment, the solution is delivered to the filter at a preferred and optimum flow rate of about 100-10,000 GPM (gallons per minute), though this value may be varied as needed in accordance with preliminary pilot studies involving the particular system under consideration and its overall capacity. Passage of the permeate through the filtration membrane typically occurs at an optimum and non-limiting membrane flux rate of about 2-20 GFD (gallons per ft2 per day]).
The precious metal may be recovered from the permeate by any number of techniques. The terms xe2x80x9crecoveryxe2x80x9d and xe2x80x9crecoveringxe2x80x9d in connection with the recovery of elemental gold and/or silver and/or other metals may comprise a number of procedures and shall not be restricted to any particular precious or nonprecious metal isolation techniques. For example, the recovery method can be by cementation (e.g., the Merrill-Crowe process), amalgamation (e.g., using an amalgamating agent such as mercury), precipitation (e.g., as a sulfide), electrolysis (e.g., electrowinning), ion exchange (e.g., solvent extraction), and/or adsorption or absorption (e.g., the activated carbon process) or any combination thereof.
After recovery of the precious metal, the permeate can be recycled to extract further precious and nonprecious metals from additional material. For example, the permeate can be passed through a second filter having a smaller pore size than the first filter to form a second retentate including at least most of the dissolved monovalent metal cyanide complexes remaining in the permeate after the recovery step and a second permeate including at least most of any water in the permeate.
The retentate can be subjected to further recovery steps to recover one or more of the metals in the dissolved multivalent metal cyanide complexes and/or the cyanide in the complexes. By way of example, the retentate can be contacted with a chelating agent and the cyanide thereafter removed from the retentate to form a cyanide depleted retentate; the metal recovered from the cyanide depleted retentate to form a barren retentate; and the barren retentate passed through a filter to form a second retentate including at least most of the chelating agent in the barren retentate and a second permeate. The retentate can be acidified to convert cyanide into HCN; the HCN removed from the acidified retentate as a gas to form a cyanide depleted retentate; the depleted retentate contacted with a base to form an electrolytic solution for an electrowinning cell; and the metal recovered in the electrowinning cell. The metal depleted retentate can be passed through a second filter to form a second retentate for recycle to the electrowinning step and a second permeate. The retentate can be contacted with an acid to precipitate the metal from the retentate as a metal cyanide compound; the metal cyanide compound dissolved in an aqueous solution to form an electrolyte solution; and the electrolyte solution subjected to electrowinning to recover the metal. Finally, the metal can be adsorbed from the retentate onto a substrate (e.g., activated carbon); the metal desorbed in an eluate solution; the eluate passed through a second filter to form a second retentate including at least most of the copper and a second permeate; and the second retentate subjected to electrowinning. Alternatively, instead of processing the metal-cyanide complex as outlined above, this material (e.g. the retentate) may be discarded in a suitable manner.
In a further embodiment of the present invention, a process for recovering the dissolved precious metal cyanide complex is provided in which the precious metal is first recovered from the solution to form a precious metal depleted solution and the precious metal depleted solution then passed through a filter to form a retentate containing at least most of the dissolved multivalent metal cyanide complex and a permeate, generally containing most of any precious metal cyanide complex remaining in the precious metal depleted solution. The recovering step can include adsorbing the precious metal and the metal in the multivalent metal cyanide complex from the solution onto a substrate; desorbing the precious metal and the metal to form an eluate solution including the dissolved precious metal and dissolved metal; and electrowinning the precious metal from the eluate solution.
The above-described separation method can offer a number of benefits. It can more effectively utilize cyanide-containing species (e.g. free cyanide ions [(CN)xe2x88x92]) by removing the multivalent metal cyanide complexes from the system and thereby reduce production costs relative to existing processes. The elimination of multivalent metal-containing species (e.g. the copper-cyanide complex) from the system can also prevent the interference of such metals with subsequent processing steps including the electrowinning and smelting stages. As a result, xe2x80x9cimpurexe2x80x9d precious metal ore (which was previously considered economically undesirable) can be processed in a cost-effective manner. It is readily applicable to a wide variety of cyanide-based treatment methods. It is highly versatile and satisfies a long-felt need in the gold processing industry. It can decrease, relative to existing processes, consumption of reagents other than cyanide including activated carbon and zinc (depending on the particular recovery system under consideration). It can reduce electricity consumption in electrowinning relative to other processes. It can conserve resources and reduce waste generation which collectively provide important environmental benefits. It can reduce, relative to other processes, the smelting time that is needed to yield an elemental gold product. It can recover nonprecious metals from the precious metal ore which can be sold at considerable economic benefit. Finally, it can produce a highly pure precious metal product dore.